Japanese Patent Application Laid-Open No. 2010-155407 discloses an ink jet head comprising: a cavity unit provided with ink passages including a nozzle row group composed of two nozzle rows and a nozzle row group composed of six nozzle rows; and a piezoelectric actuator provided with three piezoelectric layers arranged on the upper face of the cavity unit and with electrodes arranged on and under the uppermost piezoelectric layer. The above-mentioned two nozzle row groups are arranged with an interval in between. Further, in the piezoelectric actuator, an inspection electrode pattern is formed between the two nozzle row groups. The electrostatic capacitance of the piezoelectric layer is measured by using the inspection electrode pattern and then a driving potential to be applied to the electrode is determined in correspondence to the measured electrostatic capacitance.